The U.S. space agency NASA is taking the unusual step of preparing to return a space shuttle to its hangar from the launch pad because of an Atlantic tropical storm. The move would delay again a long-postponed flight to expand the International Space Station.

Only four times since 1981 have U.S. shuttles been rolled back indoors because of tropical storms and hurricanes.

But the storm called Ernesto, is threatening to reach the Florida launch site and the space shuttle Atlantis with high winds and heavy rains.

"We are firmly on the path to rollback," said shuttle launch director Mike Leinbach. He says only a significant decline in Ernesto's intensity or direction would cause NASA to keep Atlantis at the launch pad.

"It's clear in our minds that we are rolling back per plan, unless something really extraordinary happens, and we choose not to," he added.

Once the shuttle's systems are shut down, huge transport vehicles will begin pulling it back to its hangar Tuesday morning, U.S. Eastern Time. Leinbach says the maneuver could be stopped once it is under way, if the weather improves.

But if the rollback occurs, NASA would not be able to meet its September 7 launch deadline, and would have to wait until October. NASA set the deadline because a later lift-off would conflict with Russia's plans to launch a crew to the International Space Station. That crew would exchange the Soyuz emergency escape vehicle at the station for a fresher one, a trade that occurs every six months.

Russia would prefer not to postpone the exchange, because each day's delay pushes back the crew's landing with the old Soyuz in the steppes of Kazakhstan earlier each morning, from daylight to darkness. Moscow wants daylight to help guide recovery teams in that barren region.

Still, the cosmonauts and the searchers have sophisticated locator beacons and other tools that work in darkness. As a result, U.S. space officials say, they are talking to the Russians about delaying the Soyuz trade to enable a September launch of Atlantis.

NASA is eager to get the shuttle to the space station to deliver a major new piece. It has been nearly four years since the last assembly mission, because the shuttle Columbia accident in 2003 grounded all but two test flights since then. NASA must complete the complex in the 15 construction flights that remain before shuttles are retired in 2010.

The agency's space station program director, Mike Suffredini, says, all subsequent missions depend on the Atlantis flight, because it is bringing up key early components.

"If we don't do this flight, then the next ones don't get to happen, until we get this work done," he explained.